Methods of and apparatus for testing electromagnetic indicators

ABSTRACT

An electromagnetic indicator can be tested to determine whether or not it is properly positioned to the desired indicia. This is achieved by applying electrical signals to two stator windings of the electromagnetic indicator so as to induce an electrical signal in a third of the stator windings. The induced signal varies depending upon the position of the indicia of the electromagnetic indicator. The stator windings are selected such that an induced signal of predetermined phase and of predetermined magnitude is obtained for an accurately positioned indicia.

United States Patent v [151 3,639,837

Masel et al. 5] Feb. 1, 1972 54] METHODS OF AND APPARATUS FOR ReferencesCited TESTING ELECTROMAGNETIC UMTED STATES PATENTS INDICATORS 3,368,1422/1968 Bouchard et al ..340/198 X [72] Inventors: Marvln Masel, Teaneck;Albert J.

r m Tomwav both of Primary Examiner-Rudolph V. Rolinec AssistantExaminerErnest F. Karlsen Asslgnfle! The Singer p y, New York,Attorney-S. A. Giarratana and Thomas W. Kennedy 22] Filed: Sept. 23,1970 211 Appl.No.: 74,781

[5 7] ABSTRACT An electromagnetic indicator can be tested to determinewhether or not it is properly positioned to the desired indicia. [521U.S.Cl. .5. ..324/158 R, 324/74 acheved by aPP'Ymg f F F s'gnals 51 I Clwindings of the electromagnetlc indicator so as to induce an 1 v nt...G01r electrical Signal in a hird of the Stator i g The induced [58]Field of Search ..324/158 SY, 158 SM, 158 R, Signal varies dependingupon the position of the indicia f the 324/146, 55, 74; 0/ R, 324electromagnetic indicator. The stator windings are selected 323/51;336/135 such that an induced signal of predetermined phase and ofpredetermined magnitude is obtained for an accurately positionedindicia.

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(STEADY STATE) 0 INVENTORS Marv/h Mase/ Albert J Wed/aka METHODS OF ANDAPPARATUS FOR TESTING ELECTROMAGNETIC INDICATORS BACKGROUND OF THEINVENTION This invention relates to methods of and apparatus for testingelectromagnetic indicators. Accordingly, the general objects of theinvention are to provide new and improved methods and apparatus of suchcharacter.

Electromagnetic indicators which are positioned by energizing eitheroneor two out of five windings are widely used as numeric readouts for adigital computer. These indicators have the advantage of being easilyreadable under bright, ambient lighting conditions and also havemagnetic detent characteristics so that standby power is not required.They have very high reliability and long life in comparison withtungsten filament lamps, for example. They are designed for operationfrom a variety of digital computer signals. They pro vide an inherentmagnetic memory that requires no holding power between changes.Indicators can be supplied as single elements or as stacked assembliesin a single housing to present either a multiple'digit or alphanumericdisplay. In construction, the only moving part in each indicator,usually, is the readout drum, which is an integral part of the rotatingmagnetic assembly, Positioning of the readout drum is controlled byenergizing combinations of windings in the indicators fixed statorassembly. Electromagnetic indicators of the prior art feature magneticdetenting and low excitation power. They are comparatively compact andlightweight and have rapid response times. They operate in high shockand vibration environments; readout remains visible even when powerfails. No gears, cams, tapes, electrical contacts, brushes, filaments orlamps are required.

There is one significant drawback to the use of electromagneticindicators, especially where high reliability is desired, such as inaerospace applications. Prior to this invention, there has'been nosatisfactory method of or means for self-testing these indicators underactual operating conditions.

SUMMARY OF THE INVENTION Another object of this invention is to providenew and improved methods of and apparatus for self-testingelectromagnetic indicators.

It is another object of this invention to provide new and improvedmethods of and apparatus for testing electromagnetic indicators underactual operating conditions.

With these and other objects in mind, electromagnetic indicators of thetype having a permanent magnet rotor with indicia spaced thereabout andfixed stator windings in association therewith, whereby energiz'ation ofselected stator windings causes the rotor to position the indicia to acorresponding relationship, can be tested. The testing is achieved bythe application of a first periodic electrical waveform to one of thestator windings. A second periodic electrical waveform is applied to asecond of the stator windings. Both electrical waveforms are offrequencies that are sufficiently high so that the rotor does not rotateby the application of those waveforms. An electrical waveform that isinduced in a third of the stator windings is sensed and its magnitudeand phase are compared with the standard. An error signal is producedwhen either the sensed waveform is out of phase with the standard orwhen the sensed waveform is of a magnitude less than the standard.

More specifically, the invention contemplates methods of and apparatusfor simultaneously testing a plurality of electromagnetic indicators ofthe type each having a permanent magnet rotor with numerical digitsspaced thereabout, whereby energization of one or two of five selectedstator windings equally spaced cyclically about the rotor causes therotor to position the digits in a corresponding relationship. Suchtesting can be achieved by the application of a first periodicelectrical waveform at a fixed frequency rate to a corresponding statorwinding X for each indicator. A second periodic electrical waveform isapplied at the same fixed frequency rate, but out of phase with thefirst waveform, to the corresponding stator winding Y for eachindicator, both of the electrical waveforms being of frequencies thatare sufficiently high whereby the rotors do not rotate by theapplication of such waveforms. Electrical waveforms, induced in thecorresponding stator winding Z for each indicator, are sensed andcoupled periodically, at the same fixed frequency rate, to a point ofreference potential so that half-wave sampling takes place. Thehalf-wave sampled waveforms are amplified and filtered so: as to yieldDC voltages. The minimum yielded DC voltage .is compared against astandard, and an error signal is produced whenever the minimum DCvoltage is of an insufficient amplitude. The values X, Y, and Z,referred to hereinabove, are cyclically numbered, such that, to test for0 or 5, X 5, Y=3, #4; to test for l or 6, X=3, Y=l', Z=2; to test for 2or 7, X==l, Y=4, Z=5; to test for 3 or 8, X=4, Y=2, Z=3; and to test for4 or 9, X=2, Y=5, Z=l.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, advantages, and aspectsof the invention will become apparent by reference to the followingdetailed specification and drawings of a specific embodiment thereof,wherein:

FIG. l-is an electrical schematic diagram of one embodiment of theinvention;

FIG. 2 is a table indicating which one or two of the stator windings,indicated as LOGIC," should be energized for any desired numeric INDICIFIG. 3, is a table indicating, for a given INDICIA, which windings X andY are to have electrical signals applied thereto for purposes of tests,and which winding Z is to be sensed;

FIG. 4a-4e is a set of waveforms on a common time scale, illustratingfor X=5, Y=3, Z=4, and for an indicator rotor indicia position of 0," aseries (a) of voltage pluses applied to winding 3, a series (b) ofvoltage pulses applied to winding 5, a series (c) of induced voltagepulses on winding 4, a half-wave sampled-set (d) of induced pulses, andan amplified, filtered level (e)'produced from the sampled pulses;

FIG. 5a-5e is a set of waveforms, similar to those shown in FIG. 4, foran indicator rotor indicia position of 1;

FIG. 6a-6e is a set of waveforms, similar to those shown in FIG. 4, foran indicator rotor indicia position of 2;

FIG. 7a7e is a set of waveforms, similar to those shown in FIG. 4, foran indicator rotor indicia position of 3;

FIG. 8a-8e is a set of waveforms, similar to those shown in FIG. 4, foran indicator rotor indicia position of 4";

FIG. 9a-9e is a set of waveforms, similar to those shown in FIG. 4, foran indicator rotor indicia position of 5;

FIG. 10a-10e is a set of waveforms, similar to those shown in FIG. 4,for an indicator rotor indicia position of 6";

FIG. Ila-11a is a set of waveforms, similar to those shown in FIG. 4,for an indicator rotor indicia position of 7";

FIG. l2a12e is a set of waveforms, similar to those shown in flG. 4, foran indicator rotor indicia position of 8"; and

FIG. -132 is a set of waveforms, similar to those shown in FIG. 4,for anindicator rotor indicia position of 9.

Referring to FIG. I, there is shown an electromagnetic indicator 10having a rotor 11 and a plurality of stator windings l. 2, 3, 4-, 5spaced cyclically about the rotor 11. Each of the stator windings l, 2,3, 4, S is coupled to a common line 12 which is coupled to a commonpositive polarity source 13.

The electromagnetic indicator rotor 11 has a horizontal permanent magnetwhich is attracted to one of the five windings l, 2, 3, 4, 5 when one ofthe five windings is actuated; the magnet is attracted to the midpointof the two adjacent windings when those two adjacent windings areactuated. The magnet need not necessarily be horizontal; it should beoriented across the. diameter of the rotor 11.

The illustration in FIG. I of the five windings l, 2, 3, 4, 5 spacedequiangularly about the rotor 11, and the rotor 11 being shown as amagnet having a single north pole, is meant for illustrative symbolicpurposes, since many physical variations are known to the art.

In normal operation, as is well known to the prior art, a computerand/or indicator drive circuitry 14 selects one or two of five drivetransistors 16, 17, 18, 19, 20 to position the rotor 11. A table showingthe windings 1, 2, 3, 4, 5 requiring energization for each INDICIA isshown in FIG. 2.

By alternately pulsing two of the windings 1, 2, 3, 4, 5 at a high rateand by sensing the voltage induced on a third winding, a failure inpositioning the indicator rotor 11 can be detected. FIGS. 4 to 13illustrate the voltage waveforms induced in the winding 4 when windings3 and 5 are pulsed as described hereinafter, for rotor position through9, respectively. For a l-volt excitation of the indicator commonterminal 13, the maximum amplitude of the induced waveform is of theorder of one-half volt peak to peak.

From a review of FIGS. 4 to 13, it is seen that the relative phase andmagnitude of the induced waveform varies with rotor position. For 0 and5 as INDICIA (see FIGS. 4 and 9), it is noted that the induced voltageis in phase with the indicator drive transistor excitation, while for 3and 7 (see FIGS. 7 and 11), a maximum out-of-phase relationship exits.Other rotor positions show intermediate characteristics.

While alternately pulsing the proper windings and by using the pulsingwaveform to demodulate or sample the induced voltage on a properlyselected winding, a phase-sensitive DC voltage can be developed todetermine whether an indicator has failed.

Referring again to FIG. 1, there is shown, to the right of the dottedline, circuitry for self-testing the electromagnetic indicator 10.

The circuitry includes five test transistors 21, 22, 23, 24, 25, thebases of which are coupled via base resistors 27, 28, 29, 30, 31,respectively, to the computer and indicator drive circuitry 14. Theemitters of the test transistors 21, 22, 23, 24, 25 are coupled to apoint of reference potential, such as ground. The collectors of thetransistors 21, 22, 23, 24, 25 are connected to common junctionterminals 36, 37, 38, 39, 40, respectively. The stator winding 1 iscoupled via a DC blocking capacitor 41, and a serially connectedresistor 42 to the junction terminal 36. The stator winding 2 is coupledvia a DC blocking capacitor 43 and a serially connected resistor 44 tothe junction terminal 37. The stator winding 3 is coupled by a DCblocking capacitor 46 and a serially connected resistor 47 to thejunction terminal 38. Similarly, the stator winding 4 is coupled by a DCblocking capacitor 48 and a serially connected resistor 49 to thejunction terminal 39 and, likewise, the stator winding 5 is coupled viaa DC blocking capacitor 51 and a serially connected resistor 52 to thejunction terminal 40. The junction terminals 36, 37, 38, 39, 40 arecoupled, respectively, via resistors 54, 55, 56, 57, 58 to a commonjunction 59. The common junction 59 is connected to the negative inputterminal of an operational amplifier 61. The positive input terminal ofthe operational amplifier 61 is connected to a point of referencepotential, such as ground. The output of the operational amplifier 61 isconnected to its negative input terminal via a parallel connection of afeedback capacitor 62 and a resistor 63.

The output of the operational amplifier 61 is coupled to the cathode ofa diode 64 whose anode is connected to a junction terminal 66. In asimilar fashion, other similar test circuits for other electromagneticindicators (not shown) are coupled via diodes 64', 64", etc., to thecommon junction terminal 66. The junction terminal 66 is coupled via avoltage dropping resistor 67 to a point of positive voltage source +V.The junction terminal 66 is also connected to a comparing amplifier 68which has one input coupled to a point of reference voltage and has astrobe" terminal adapted to be actuated periodically in an interrogatingmanner. The output of the amplifier 68 is coupled to means forindicating an error. Such means may be, for example, a light, an audiblealarm, or can be an instruction to the computer to indicate that anerror has taken place.

In operation, assume that the electromagnetic indicator 11 should be,and actually is, indicating a 0." This was achieved by previouslydriving the stator winding 1, as indicated by the table shown in FIG. 2.To test for an indicia of 0" as indicated in FIG. 3, the winding 5 isdriven by a chain of pulses (FIG. 4a) coincidentally with theapplication ofa chain of pulses (FIG. 41)) applied to the winding 3. Thevoltage induced in the winding 4 is depicted in FIG. 4c.

The windings 3 and 5 are alternately pulsed by turning on transistors18, 20 with square wave pulses, while the winding 4 is read. Whenhalf-wave sampled (FIG. 4d), amplified, filtered, and integrated, avoltage level (FIG. 4e) will be produced which exceeds a fixed thresholdvalue, thus indicating an acceptable indicia. Such will be the case whenthe electromagnetic indicator is functioning properly.

It is not critical whether the transistors 18, 20 are turned on and offwith square waves or sinusoidal waves, but simply that they are turnedon and off. The demodulating test transistor 30 is turned on and ofi" insynchronism with the transistor 18. When the indicator ll properly is atthe 0" position, the output from the winding 4 tends to be sinusoidal(FIG. 4c) in phase with the demodulating voltage, the current being inphase with the driving voltage. However, the positive halves of thewaves are grounded by the demodulating transistor 30, so that all thatappears at the input of the operational amplifier 61, is, in effect,negative half-waves of current. The negative half-waves of current, whenfed into the inverting operational amplifier 61, due to the filteringfeedback capacitor 62, are integrated to a particular positive DCvoltage. This positive DC voltage, when the indicator 11 is at theproper position, is at a maximum magnitude, as shown in FIG. 4. In oneexample, the magnitude was in the neighborhood of 1.6 volts.

However, when the electromagnetic indicator 11 does not functionproperly, in that the erroneous indicia was being indicated when itshould read 0 for example, then one of the sets of the waveforms asshown in FIGS. 5 through 13 occur. When the indicator is at an erroneousposition, a voltage will be read from the amplifier 61 output at amagnitude substantially less than 1.6 volts. The 1.6 volts output, orless, are grouped together via the diodes 64, 64, 64" (they areisolating diodes) and the resistor 67 to the +V source, to an amplifier68 which acts as a threshold detector, so that the input thereto for acorrect level should be about 2.1 volts, due to the voltage drop acrossthe diodes 64, 64', 64". This 2.1 volts should be slightly higher thanthe V ref, which is in the neighborhood of 1.9 to 2.0 volts-Theamplifier 68 is strobed, and a failure is indicated by a high level fromthe output of the amplifier 68.

In the example given, wherein the stator winding 4 is being sensedduring a test interval, the test transistors 21, 22, 23, 25 other thanthe one associated with the stator winding 4, are so biased that theyconduct, whereby the junctions 36, 37 38, 40 are effectively connectedto ground. The fourth test transistor 24 is periodically clocked at thesame frequency rate by the computer and indicator drive circuitry 14 sothat the waveform from the stator winding 4 is half-wave chopped, orsampled, to provide a train of half-wave voltage pulses. These voltagepulses are applied via the resistor 57 to the common terminal 59 to thenegative input terminal of the operational amplifier 61. An integratingaction takes place in the operation amplifier 61 due to the capacitor62. More particularly, the capacitor 62 filters the high frequencyapplied thereto so as to convert the pulsating half-wave sampled inputto a phasesensitive DC voltage output. The output of the amplifier 61,in a few cycles, appears as a steady-state voltage. This steadystatevoltage, in a desired embodiment, is comparatively high whereby thediode 64 does not conduct. When the diode 64 does not conduct, and whenother indicators are testing properly, their corresponding diodes 64 and64" do not conduct. When the diodes 64, 64, 64" do not conduct, ascurrent flows through the resistor 67 and, hence, the voltage at thecommon terminal 66 is at the level +V. The voltage +V at the commonterminal 66 is slightly higher than the voltage V ref 0 that, upon beingstrobed, the amplifier 68 provides an indication that no error takesplace. This failure indicating voltage can be coupled to a failureindicator, a light, an alarm, or

otherwise coupled back to the computer system to indicate a failure.

In the event, however, that an error did occur, the induced signal froma sensed stator winding would be of insufficient magnitude and/orimproper phase, such that, upon the demodulating action of applying thepulsating waveform to the corresponding test transistor, the demodulatedvoltage at the common junction 39 would not have an average magnitude ofa sufficiently high value. Assuming that, when deleting half of a cyclethat either the signal is too weak that remains or is of improper phase,then the operational amplifier 61 produces a signal at the outputthereof which is somewhat indicative or a function of the averagevoltage that appears at the common junction terminal 59. This averagevoltage would be of a comparatively low value such that conduction takesplace through the diode 64. When the diode 64 conducts, current is drawnthrough the resistor 67, causing a voltage drop thereacross, whereby thevoltage at the common junction 66 drops in value. Hence, when theamplifier 68 is strobed to check for comparison between V ref and thevoltage at the terminal 66, a suitable indication is given that an errorhas taken place.

The signals from each of five windings ll, 2, 3, 4, 5 are coupledthrough the DC blocking capacitors 41, 43, 46, 48, 51 and the seriesresistors 42, 44, 47, 49, 52 to the summing junction 59 of the DCoperational amplifier 61. Four of the transistors 21, 22, 23, 24, 25 areforced to conduct during the self-testing operation, while the fifthtransistor is turned on and off in synchronism with the pulsing currentof two selected windings. As an example, when the computer 14 haswritten" the indicia into the indicator by turning the winding 1 on forabout a half-second, and then removing power'from the winding 1, it ispossible to subsequently check whether the indicator rotor is at theproper position by pulsing the windings 3 and 5 at a 5,000 pulse persecond rate while pulsing the built-in test equipment" transistor 24 onand off in synchronism with the winding drive transistor 20. The builtintest equipment transistors 21, 22, 23, 25 are forced to conductcontinuously during the self-test operation. The current flowing intothe summing junction 59 of the operational amplifier 61 produces thephase-sensitive DC voltage at the amplifier 61 output. The capacitor 62,in the feedback path of the operational amplifier 61, filters the 5,000hertz fundamental and higher harmonics with a time constant of about onefivehundredths second. The scaling that was used in one embodimentinvolved an amplifier feedback resistor 63 times the sum of the twoinput resistors 49 and 57. For 0 or 5 output indicia, the amplifieroutput is about 1.6 volts. The diodes 64, 64, 64" connecting the outputof the operational amplifiers 61 with a test circuitry fora large numberof other indicators select the lowest indicator voltage for comparisonwith the voltage reference. Thus, when the test circuitry is so scaledthat a correct rotor position causes l.6-volt output of the amplifier61, the amplifier 68 input should be about one diode drop higher orabout 2.1 volts. A reference of about 1.5 volts as a failure criteriacan be selected, allowing for various tolerances. When any one of theamplifier 61 outputs, then is less than, say, 1 volt and the comparingamplifier 68 is strobed, a signal can be sent to a failure indicator. Byobserving the table shown in FIG. 3, it is apparent that two indicia 180apart, for example, 0 or 5, cannot be distinguished reliably. Thus, forexample, when the computer commands the indicator 10 to show 0" and theindicator 10 was mechanically stuck at 5, the self-test circuitry doesnot indicate a failure. in practice, however, this limitation is notvery serious, provided that the test is repeated either after eachupdate of the indicators 10 or at least at occasional intervals. Thus,when after being positioned at 0, the command were subsequently changedto 1, a mechanical bind at 5" would immediately indicate a failure. Ofcourse, as part of a preflight checkout, the computer 14 could sequenceall the indicators 10 through 0," ll," 2," etc., positions. Subsequenttesting would continue as described hereinbefore.

The self-test methods described also operate successfully in case ofloss of power to the common of the indicator 11 or due to open or shortof either the indicator windings, wiring, or the drive transistors. Lossof power causes failure indication invariably. Other failures aredetected with less than percent probability. Detection becomes virtuallycertain as described earlier when the self-test is repeated after newcomputer commands.

In summary, by pulsing the rotors ll of electromagnetic indicators 10 inthe manner described, an induced voltage on one of the stator windingswhich is of known polarity and magnitude is obtained when the rotor 11is at the proper position. When the position is incorrect, othervoltages are obtained which can be sensed by the demodulation meansdescribed. Therefore, built-in test equipment can be applied toindicators 11 in an actual system without modifying the design of theindicators ll. Advantageously, the indicators need not be visually readby a person; the error indication is automatic, independent of visualreadings of the indicators. The indicators may be at any position; theyneednt be set up to particular positions for a test. Testing can becontinuously performed in an on-time" basis, since it is not apparentthat testing takes place; that is, the indicator does not appear tomove. Hence, uninterrupted service and nondestructive testing areachieved without disturbing the operator.

It is to be understood that the embodiment described is merelyillustrative and is not to be considered to be limiting of the inventionin any manner whatsoever. As illustrated herein, NPN-trarisistors areused; however, of course, vacuum tubes, PNP-transistors, or other typesof drive circuitry can be utilized without departing from the spirit andscope of the invention. The operational amplifier 61 may be of the 709type, and the comparing amplifier 68 may be of the 710 type, by way ofexamples. Indicia other than numerical digits can be used, such asalphanumeric symbols, punctuation, etc. it is to be understood that theabove-described arrangement of apparatus and methods are illustrative ofapplications of the principles of the invention and other modificationsmay be made without departing therefrom.

What is claimed is:

l. A method of testing an electromagnetic indicator of the type having apermanent magnet rotor with indicia spaced thereabout and fixedstatorwindings in association therewith, whereby energization of selectedstator windings causes said rotor to position said indicia in acorresponding relationship, said method comprising, for a given one ofsaidindicia, the steps of applying a first periodic electrical waveformto a first of said stator windings;

applying a second periodic electrical waveform to a second of saidstator windings;

said electrical waveforms being of frequencies sufficiently high wherebysaid rotor does not rotate by application of said waveforms;

sensing the electrical waveform induced in a third of said statorwindings; comparing the magnitude and phase of the sensed waveform witha standard; and

producing an error signal when either the sensed waveform is out ofphase with said standard, or when the sensed waveform has a magnitudeless than said standard.

2. The method as recited in claim 1 wherein said first and said secondwaveforms are square waves out of phase with each other, and occur at afixed frequency rate, and wherein said sensed waveform is coupled to apoint of reference potential at said fixed frequency rate.

3. A method of testing an electromagnetic indicator of the type having apermanent magnet rotor with indicia spaced thereabout, wherebyenergization of selected stator windings causes said rotor to positionsaid indicia in a corresponding relationship, said method comprising,for a given one of said indicia, the steps of applying a first periodicelectrical square waveform at a fixed frequency rate to a first of saidstator windings;

applying a second periodic electrical square waveform at said fixedfrequency rate, out of phase with said first waveform, to a second ofsaid stator windings, said electrical waveforms being of frequenciessufficiently high whereby said rotor does not rotate by application ofsaid waveforms;

sensing the electrical waveform induced in a third of said statorwindings;

coupling said sensed waveform, periodically at said fixed frequencyrate, to a point of reference potential, whereby half-wave samplingtakes place;

amplifying and filtering the half-wave sampled waveform so as to yield aDC voltage;

comparing the yielded DC voltage versus a standard; and

producing an error signal when the yielded DC voltage is of aninsufficient magnitude.

4. A method of simultaneously testing a plurality of electromagneticindicators of the type, each having a permanent magnet rotor with 10numerical digits spaced thereabout whereby energization of one or two offive selected stator windings equally spaced cyclically about the rotorcauses said rotor to position said digits in a correspondingrelationship, said method comprising the steps of applying a firstperiodic electrical square waveform at a fixed frequency rate to thestator winding X for each indicator;

applying a second periodic electrical square waveform at said fixedfrequency rate, out of phase with said first waveform, to the statorwinding Y for each indicator,

said electrical waveforms being of frequencies sufficiently high wherebysaid rotors do not rotate by application of said waveforms;

sensing the electrical waveforms induced in the stator winding Z foreach indicator;

coupling said sensed waveforms, periodically at said fixed frequencyrate, to a point of reference potential, whereby half-wave samplingtakes place;

amplifying and filtering the half-wave sampled waveforms so as to yieldDC voltages;

comparing the minimum yielded DC voltage versus a standard; and

producing an error signal when the minimum yielded DC voltage is of aninsufficient magnitude, wherein X, Y, and Z are cyclically numbered suchthat, to test for or 5, X=5, Y=3, Z=4; to test for l or 6, X=3, Y=l,Z=2; to test for 2 or 7, X=l, Y=4, Z=; to test for 3 or 8, X=4, Y=2,Z=3; and to test for 4 or 9, X=2, Y=5, Z=l.

5. Apparatus for testing an electromagnetic indicator of the type havinga permanent magnet rotor with indicia spaced thereabout and fixed statorwindings in association therewith, whereby energization of selectedstator windings causes said rotor to position said indicia in acorresponding relationship, said apparatus comprising, for a given oneof said indicia,

means for applying a first periodic electrical waveform to a first ofsaid stator windings;

means for applying a second periodic electrical waveform to a second ofsaid stator windings;

said electrical waveforms being of frequencies sufficiently high wherebysaid rotor does not rotate by application of said waveforms;

means for sensing the electrical waveform induced in a third of saidstator windings;

means for comparing the magnitude and phase of the sensed waveform witha standard; and

means for producing an error signal when either the sensed waveform isout of phase with said standard, or when the sensed waveform has amagnitude less than said standard.

6. The apparatus as recited in claim 5 wherein said first and saidsecond waveforms are square waves out of phase with each other, andoccur at a fixed frequency rate, and wherein said sensed waveform iscoupled to a point of reference potential at said fixed frequency rate.

7. Apparatus for testing an electromagnetic indicator of the type havinga permanent magnet rotor with indicia spaced t ereabout, wherebyenergiza ion of selected stator wmdmgs causes said rotor to positionsaid indicia in a corresponding relationship, said apparatus comprising,for a given one of said indicia,

means for applying a first periodic electrical square waveform at afixed frequency rate to a first of said stator windings;

means for applying a second periodic electrical square waveform at saidfixed frequency rate, out of phase with said first waveform, to a secondof said stator windings,

said electrical waveforms being of frequencies sufficiently high wherebysaid rotor does not rotate by application of said waveforms;

means for sensing the electrical waveform induced in a third of saidstator windings;

means for coupling said sensed waveform, periodically at said fixedfrequency rate, to a point of reference potential, whereby half-wavesampling takes place;

means for amplifying and filtering the half-wave sampled waveform so asto yield a DC voltage;

means for comparing the yielded DC voltage versus a standard; and

means for producing an error signal when the yielded DC voltage is of aninsufficient magnitude.

8. Apparatus for simultaneously testing a plurality of electromagneticindicators of the type, each having a permanent magnet rotor with 10numerical digits spaced thereabout whereby energization of one or two offive selected stator windings equally spaced cyclically about the rotorcauses said rotor to position said digits in a correspondingrelationship, said apparatus comprising means for applying a firstperiodic electrical square waveform at a fixed frequency rate to thestator winding X for each indicator;

means for applying a second periodic electrical square waveform at saidfixed frequency rate, out of phase with said first waveform, to thestator winding Y for each indicator,

said electrical waveforms being of frequencies sufficiently high wherebysaid rotors do not rotate by application of said waveforms;

means for sensing the electrical waveforms induced in the stator windingZ for each indicator;

means for coupling said sensed waveforms, periodically at said fixedfrequency rate, to a point of reference potential, whereby half-wavesampling takes place;

means for amplifying and filtering the half-wave sampled waveforms so asto yield DC voltages;

means for comparing the minimum yielded DC voltage versus a standard;and

means for producing an error signal when the minimum yielded DC voltageis of an insufficient magnitude, wherein X, Y, and Z are cyclicallynumbered such that, to test for O or S, X=5, Y=3, Z=4; to test for l or6, X=3, Y=l Z=2; to test for 2 or 7, X=l ,'Y=4, Z=5; to test for 3 or 8,X=4, Y=2, Z=3; and to test for 4 or 9, X=2, Y=5, Z=1.

9. Apparatus as recited in claim 8 wherein said means for amplifying andfiltering the half-wave sampled waveforms, for each electromagneticindicator, includes an operational amplifier, and 1 a resistor and acapacitor, in parallel, coupled across the output and an input of saidamplifier.

1. A method of testing an electromagnetic indicator of the type having apermanent magnet rotor with indicia spaced thereabout and fixed statorwindings in association therewith, whereby energization of selectedstator windings causes said rotor to position said indicia in acorresponding relationship, said method comprising, for a given one ofsaid indicia, the steps of applying a first periodic electrical waveformto a first of said stator windings; applying a second periodicelectrical waveform to a second of said stator windings; said electricalwaveforms being of frequencies sufficiently high whereby said rotor doesnot rotate by application of said waveforms; sensing the electricalwaveform induced in a third of said stator windings; comparing themagnitude and phase of the sensed waveform with a standard; andproducing an error signal when either the sensed waveform is out ofphase with said standard, or when the sensed waveform has a magnitudeless than said standard.
 2. The method as recited in claim 1 whereinsaid first and said second waveforms are square waves out of phase witheach other, and occur at a fixed frequency rate, and wherein said sensedwaveform is coupled to a point of reference potential at said fixedfrequency rate.
 3. A method of testing an electromagnetic indicator ofthe type having a permanent magnet rotor with indicia spaced thereabout,whereby energization of selected stator windings causes said rotor toposition said indicia in a corresponding relationship, said methodcomprising, for a given one of said indicia, the steps of applying afirst periodic electrical square waveform at a fixed frequency rate to afirst of said stator windings; applying a second periodic electricalsquare waveform at said fixed frequency rate, out of phase with saidfirst waveform, to a second of said stator windings, said electricalwaveforms being of frequencies sufficiently high whereby said rotor doesnot rotate by application of said waveforms; sensing the electricalwaveform induced in a third of said stator windings; coupling saidsensed waveform, periodically at said fixed frequency rate, to a pointof reference potential, whereby half-wave sampling takes place;amplifying and filtering the half-wave sampled waveform so as to yield aDC voltage; comparing the yielded DC voltage versus a standard; andproducing an error signal when the yielded DC voltage is of aninsufficient magnitude.
 4. A method of simultaneously testing aplurality of electromagnetic indicators of the type, each having apermanent magnet rotor with 10 numerical digits spaced thereaboutwhereby energization of one or two of five selected stator windingsequally spaced cyclically about the rotor causes said rotor to positionsaid digits in a corresponding relationship, said method comprising thesteps of applying a first periodic electrical square waveform at a fixedfrequency rate to the stator winding X for each indicator; applying asecond periodic electrical square waveform at said fixed frequency rate,out of phase with said first waveform, to the stator winding Y for eachindicator, said electrical waveforms being of frequencies sufficientlyhigh whereby said rotors do not rotate by application of said waveforms;sensing the electrical waveforms induced in the stator winding Z foreach indicator; coupling said sensed waveforms, periodically at saidfixed frequency rate, to a point of reference potential, wherebyhalf-wave sampling takes place; amplifying and filtering the half-wavesampled waveforms so as to yield DC voltages; comparing the minimumyielded DC voltage versus a standard; and producing an error signal whenthe minimum yielded DC voltage is of an insufficient magnitude, whereinX, Y, and Z are cyclically numbered such that, to test for 0 or 5, X 5,Y 3, Z 4; to test for 1 or 6, X 3, Y 1, Z 2; to test for 2 or 7, X 1, Y4, Z 5; to test for 3 or 8, X 4, Y 2, Z 3; and to test for 4 or 9, X 2,Y 5, Z
 1. 5. Apparatus for testing an electromagnetic indicator of thetype having a permanent magnet rotor with indicia spaced thereabout andfixed stator windings in association therewith, whereby energization ofselected stator windings causes said rotor to position said indicia in acorresponding relationship, said apparatus comprising, for a given oneof said indicia, means for applying a first periodic electrical waveformto a first of said stator windings; means for applying a second periodicelectrical waveform to a second of said stator windings; said electricalwaveforms being of frequencies sufficiently high whereby said rotor doesnot rotate by application of said waveforms; means for sensing theelectrical waveform induced in a third of said stator windings; meansfor comparing the magnitude and phase of the sensed waveform with astandard; and means for producing an error signal when either the sensedwaveform is out of phase with said standard, or when the sensed waveformhas a magnitude less than said standard.
 6. The apparatus as recited inclaim 5 wherein said first and said second waveforms are square wavesout of phase with each other, and occur at a fixed frequency rate, andwherein said sensed waveform is coupled to a point of referencepotential at said fixed frequency rate.
 7. Apparatus for testing anelectromagnetic indicator of the type having a permanent magnet rotorwith indicia spaced thereabout, whereby energization of selected statorwindings causes said rotor to position said indicia in a correspondingrelationship, said apparatus comprising, for a given one of saidindicia, means for applying a first periodic electrical square waveformat a fixed frequency rate to a first of said stator windings; means forapplying a second periodic electrical square waveform at said fixedfrequency rate, out of phase with said first waveform, to a second ofsaid stator windings, said electrical waveforms being of frequenciessufficiently high whereby said rotor does not rotate by application ofsaid waveforms; means for sensing the electrical waveform induced in athird of said stator windings; means for coupling said sensed waveform,periodically at said fixed frequency rate, to a point of referencepotential, whereby half-wave sampling takes place; means for amplifyingand filtering the half-wave sampled waveform so as to yield a DCvoltage; means for comparing the yielded DC voltage versus a standard;and means for producing an error signal when the yielded DC voltage isof an insufficient magnitude.
 8. Apparatus for simultaneously testing aplurality of electromagnetic indicators of the type, each having apermanent magnet rotor with 10 numerical digits spaced thereaboutwhereby energization of one or two of five selected stator windingsequally spaced cyclically about the rotor causes said rotor to positionsaid digits in a corresponding relationship, said apparatus comprisingmeans for applying a first periodic electrical square waveform at afixed frequency rate to the stator winding X for each indicator; meansfor applying a second periodic electrical square waveform at said fixedfrequency rate, out of phase with said first waveform, to the statorwinding Y for each indicator, said electrical waveforms being offrequencies sufficiently high whereby said rotors do not rotate byapplication of said waveforms; means for sensing the electricalwaveforms induced in the stator winding Z for each indicator; means forcoupling said sensed waveforms, periodically at said fixed frequencyrate, to a point of reference potential, whereby half-wave samplingtakes place; means for amplifying and filtering the half-wave sampledwaveforms so as to yield DC voltages; means for comparing the minimumyielded DC voltage versus a standard; and means for producing an errorsignal when the minimum yielded DC voltage is of an insufficientmagnitude, wherein X, Y, and Z are cyclically numbered such that, totest for 0 or 5, X 5, Y 3, Z 4; to test for 1 or 6, X 3, Y 1, Z 2; totest for 2 or 7, X 1, Y 4, Z 5; to test for 3 or 8, X 4, Y 2, Z 3; andto test for 4 or 9, X 2, Y 5, Z
 1. 9. Apparatus as recited in claim 8wherein said means for amplifying and filtering the half-wave sampledwaveforms, for each electromagnetic indicator, includes an operationalamplifier, and a resistor and a capacitor, in parallel, coupled acrossthe output and an input of said amplifier.